From antimatter to mirror matter

The idea of a 'mirror world' was first suggested in 1956 by Chinese-American physicists Chen Ning Yang and Tsung Dao Lee. First, a tiny dose of particle physics, before you can enter their 'mirror world'.

The universe is held together by four types of fundamental forces - gravity, electromagnetism, the strong force, and the weak force - which are transmitted or 'mediated' by the exchange of elementary particles. The gravitational force, or gravity, is the long-range force responsible for the attraction existing between all matter: it holds you to the ground and Earth in its orbit. Its range is infinite. The electromagnetic force is the attraction and repulsion between charged particles: it enables a light bulb to glow and a magnet to stick to your fridge. Its range is also infinite. The strong force is the 'glue' that holds together an atomic nucleus: it binds quarks to make protons and neutrons. The weak force is also a kind of nuclear force: it causes elementary particles to shoot out of the atomic nucleus during the radioactive decay of elements such as uranium. The range of the strong and the weak force is extremely short. The electromagnetic, the weak and the strong forces are very similar and are very well understood by physicists, but gravity is still a mystery, and little is known of its relation to the other forces.

The existence of anti-matter leads to the idea of symmetry, that is, every particle has a mirror-like twin. An anti-particle would look just like the ordinary particle, except that left would be switched with right. Physicists call it reversing the parity (parity is just a sexy word for left-right or mirror symmetry). Symmetry also applies to laws of physics such as the rules governing the interaction of elementary particles. All the original laws should continue to work in the same way: whatever could happen in the real world would also happen in the anti-matter world.

But nature's symmetry is flawed. Certain interactions of elementary particles always produce a particle always spinning in the same direction. For example, when an atom emits a neutrino it always spins in the same direction - left-handedly (if it were coming towards you, you would see it spinning clockwise). Reflected in a mirror, however, a neutrino would be right-handed (it would always spin anti-clockwise). In contrast, electrons can spin in both directions. As many elementary particles display a preference for left over right, the universe seems left-handed. Why? Physicists do not know.

In 1956 Yang and Lee suggested that the evidence for left-right symmetry was weak in interactions involving the weak force (which led Wolfgang Pauli, who had dreamed up neutrinos in 1930, to lament: 'I cannot believe God is a weak left-hander.'). This prediction was soon confirmed experimentally by other physicists. Mirror symmetry or parity was now dead. Asymmetry was the new king. The discovery of asymmetry won Yang and Lee the Nobel Prize in physics just a year later.

Like nifty accountants, Yang and Lee had to balance the books. They proposed a way to restore perfect left-right symmetry to nature: every right-handed particle might have a left-handed particle, and vice-versa. This means that in addition to the anti-matter world, there might also exist a mirror world. In the mirror world, all neutrinos would be right-handed. Considered together, the real world and the mirror world would restore the symmetry that appears to be lacking in each.

Welcome to the mirror world - a world of mirror planets, mirror stars and even mirror life, all governed by mirror forces. This world is as fanciful as the one Alice entered Through the Looking Glass.

In this world, particles are right-hand or mirror images of ordinary particles. They also have the same mass as their ordinary counterparts. Thus, one force that acts on both ordinary matter and mirror matter is gravity. But there would not be any interaction between ordinary matter and mirror matter through nature's other three forces - the electromagnetic, the strong and the weak. We should be able to detect gravitational force when mirror matter comes near ordinary matter. The detec tion of this force would betray the presence of invisible mirror matter. The testability of this idea takes mirror matter out of the realm of science fiction into reality.

Because we are made of ordinary matter, we can neither see nor smell mirror matter (or our mirror matter twins, even if they were dressed in their brightest mirror matter clothes and soaked in mirror matter perfume). If you did encounter your mirror matter twin, you would pass right through him or her. You would also be invisible to your twin.

No mirror matter has yet been discovered or made in the laboratory, but neutrinos provide a misty glimpse of the mirror world. Neutrinos are the most pervasive elementary particles in the universe. There are about 50 billion neutrinos for every electron; they are everywhere but they cannot be seen and rarely interact with matter. Tens of thousands pass through our body every second. They have no charge and, although previously thought to have no mass at all, they are now believed to have a small amount of mass. There are three known types of neutrino - muon, tau and electron - and they are all created in the centre of the Sun, in supernovas and in the cosmic rays hitting the upper atmosphere. (In his famous book The Quark and the Jaguar, Murray Gell-Mann writes that the neutrinos produced by the Sun 'reach the surface of the earth by raining down on us during the day, but at night they come up at us through the earth'. This aspect of neutrino behaviour inspired writer John Updike to write a poem entitled 'Cosmic Gall'. An excerpt: 'The earth is just a silly ball / To them, through which they simply pass, / Like dustmaids down a drafty hall / Or photons through a sheet of glass.')

Physicists have calculated the number of electron neutrinos that should reach Earth from the Sun. But they have actually detected fewer of these than predicted. Recent experiments have shown that neutrinos can change from one type to another. Some types of neutrinos are not spotted by neutrino detectors, which explains the discrepancy. The proponents of mirror matter solve the puzzle of missing solar neutrinos by suggesting the existence of a fourth type of neutrino -mirror neutrinos. These are so ghostly that they don't make their presence known to bewildered physicists.

There is more good news for those who believe in the existence of mirror matter. And it comes from a particle called orthopositronium. Positronium is like a hydrogen atom, but instead of an electron orbiting a proton, an electron orbits a positron, its anti-matter counterpart. If the spin of the electron and the spin of the positron point in the same direction, the atom is known as orthoposi-tronium. In 1986, Harvard physicist and Nobel Laureate Sheldon Glashow suggested that orthopositronium could oscillate between mirror and ordinary orthopositronium - jumping back and forth through the mirror.

Orthopositronium is ephemeral; it lasts a mere 142 nanoseconds before its components annihilate each other in a burst of tiny energy in the form of three undetectable photons. However, in the 1990s, when physicists made a batch of orthopositronium, they found that its lifetime is shorter than 142 nanoseconds. In 2000,

Robert Foot of the University of Melbourne and Sergei Gninenko of CERN suggested that mirror ortho-positronium could explain the discrepancy. This could be due to orthopositronium changing fleetingly into its mirror matter form and then back again. The mirror orthopositronium would go undetected, and that could account for the shorter lifetime measurements.

Although mirror matter is expected to interact with ordinary matter only through gravity, recent experiments suggest a small electromagnetic attraction between mirror and ordinary particles. This coupling probably comes from the tiny electric charge that mirror electrons and protons are believed to have. This charge is about a millionth that of their ordinary counterparts. The tiny electromagnetic interaction between mirror particles and ordinary particles, if it exists, has interesting implications. It would make mirror stars visible if they had some embedded ordinary matter. This interaction would also be sufficient to heat up a body of mirror matter if it entered Earth's atmosphere. And that's where Tunguska enters the mirror world.